Like any spectroscopic method, the usefulness of 31P NMR as a quantitative analytical technique depends upon establishing whether the areas under the phosphorus peaks are proportional to the relative number of 31P nuclei in the phosphorus compound in the sample. The aim of this study was to compare the 31P NMR estimates of PCr, Pi and pH in the isolated rat heart with those determined from enzymatic procedures. The major findings of the study were (1) ATP and PCr are equally visible in the isolated working rat heart, (2) the NMR estimate of intracellular pH and enzymatic tissue bicarbonate method (Dobson et al., Anal Biochem, in press.) agree to within 0.1 pH unit, and (3) with the exception of glucose-perfused hearts at high work, the cytosolic Pi content calculated by either the NMR or the near-equilibrium method was 30 to 70% of the total tissue Pi content indicating that a considerable fraction of the total was compartmented, presumably in the mitochondria.